Issue 5773 - use ES6 for stylesheets and cssRules.

lib/content/elemHideEmulation.js

Index: lib/content/elemHideEmulation.js
===================================================================
--- a/lib/content/elemHideEmulation.js
+++ b/lib/content/elemHideEmulation.js
@@ -406,35 +406,29 @@
     let elementFilters = [];
 
     let cssStyles = [];
 
     let stylesheetOnlyChange = !!stylesheets;
     if (!stylesheets)
       stylesheets = this.document.styleSheets;
 
-    // Chrome < 51 doesn't have an iterable StyleSheetList
-    // https://issues.adblockplus.org/ticket/5381
-    for (let i = 0; i < stylesheets.length; i++)
+    for (let stylesheet of stylesheets)
     {
-      let stylesheet = stylesheets[i];
       // Explicitly ignore third-party stylesheets to ensure consistent behavior
       // between Firefox and Chrome.
       if (!this.isSameOrigin(stylesheet))
         continue;
 
       let rules = stylesheet.cssRules;
       if (!rules)
         continue;
 
-      // Chrome < 51 doesn't have an iterable CSSRuleList
-      // https://issues.adblockplus.org/ticket/5773
-      for (let j = 0; j < rules.length; j++)
+      for (let rule of rules)
       {
-        let rule = rules[j];
         if (rule.type != rule.STYLE_RULE)
           continue;
 
         cssStyles.push(stringifyStyle(rule));
       }
     }
 
     let patterns = this.patterns.slice();